1. Field of the Invention
This invention relates to the production of silicone coated expanded perlite, and more particularly to a method of coating extremely lightweight particles of expanded perlite virtually simultaneously with their expansion from perlite ore.
The present invention provides an expanded perlite coated with a cured hydrophobic material such as polydimethyl siloxane in a highly efficient manner. Perlite is an ore of volcanic origin that has the unusual characteristic of expanding to about 4-20 times its original volume upon quickly heating within its softening-temperature range (generally 760.degree.-1100.degree. C.).
2. Description of the Prior Art
Processes for expanding perlite are disclosed in U.S. Pat. No. 2,455,666; 2,501,699; 2,572,483; 2,621,160; and 3,097,832 and are otherwise generally known in the art. In recent years, interest has developed in extremely lightweight and in finer grades of expanded perlite having very low bulk densities e.g. less than about 160 kg/m.sup.3, and increased surface areas, e.g. top particle size of around 150 micrometers or less, for use as a filler in paints, textures, joint compounds, plastics and the like. Furthermore the surfaces of expanded perlite particles contain many fine cracks and fissures. The production of these lower bulk weight and finer particle size perlite materials results in increased fissuring of the particles. When placed in contact with water, such as in aqueous dispersion paints, textures and joint compounds, the water penetrates the cracks and fissures and enters into the interior air filled cavities of the perlite, causing it to lose its lightweight characteristic. Further, the entrained water may be detrimental to processing plastics or other formulations which might otherwise use this lightweight filler.
Applying coatings on expanded perlite particles to seal the surfaces thereof is not per se unique to the present invention. U.S. Pat. No. 3,769,065 discloses a process of coating expanded in a prior separate operation by applying an aqueous acid solution to the particles, then applying an aqueous sodium silicate solution followed by another acid treatment, and drying the coated particles. U.S. Pat. Nos. 4,183,980 and 4,255,489 disclose a strengthened coated expanded perlite obtained by contacting the previously expanded perlite with an emulsion of polysiloxane, drying the coated particles as in a moving stream of hot gas, and then a further extended heat treatment in an oven at a temperature of 288.degree. -454.degree. C. to cure and strengthen the polysiloxane.
Other means of coating expanded perlite separate and apart from the production of the expanded perlite are known, such as vat soaking in a coating solution followed by draining, drying and curing; and pouring or spraying onto a tumbling bed followed by drying and curing. Coatings of other filler materials are also known, with U.S. Pat. No. 4,141,751, for example, disclosing spraying either a silane or titanate coupling agent onto fine particulate inorganic solids, while agitating those solids in a high intensity mixer to develop shearing friction temperatures for curing, and drying the coated particles. U.S. Pat. No. 2,730,841 discloses the production of silicone coated glass beads by spraying a siloxane dispersion onto hot glass beads after they have been separated from formation exhaust gases, and allowing the coated beads to cure from 2 to 24 hours on exposure to ambient air.